How flying reptiles rose

How did a giant flying reptile get off the ground? It's not a simple question: A computerized analysis of pterosaur fossils and modern-day bird bones shows that the biggest pterosaurs couldn't simply lift off into the air like a bird, because their hind legs were too weak.

The researcher behind the analysis says the forelimbs were much stronger - so much stronger, in fact, that the creatures must have used their "arms" as well as their legs to propel their leap into flight. But will that claim fly with other experts? That remains to be seen.

The analysis is published in a special issue of the German-based journal Zitteliana, and is the subject of a news release today from the Johns Hopkins University School of Medicine.

Pterosaurs died off 65 million years ago in the same cataclysm that killed off the dinosaurs on land and plesiosaurs at sea. So how can anyone possibly know how they took off? Michael Habib, a researcher at the medical school's Center for Functional Anatomy and Evolution, came up with a computerized model for pterosaur flight dynamics by comparing data about bone strength for three pterosaur species (Dorygnathus, Zhejiangopterus and Anhanguera ... small, medium and large) with readings from 155 bird limb specimens.

Birds are built to leap into the air using their legs, and then flap their wings for takeoff. But Habib found that pterosaurs were built differently.

"The difference between pterosaurs and birds with regard to critical mechanical properties is very, very large, especially when you're talking about the big pterosaurs," he said in the news release. "As the size gets bigger, the difference gets bigger, too."

The model indicated that Anhanguera couldn't possibly launch itself using its hind legs alone. That led Habib to suspect that the biggest pterosaurs folded their wings and balanced on their "knuckles" to walk as well as to push themselves off for flight. He envisioned the takeoff procedure as a leap-frogging long jump. "Then, with wings snapping out, off they'd fly."

"Using all four legs, it takes less than a second to get off of flat ground, no wind, no cliffs," he said. "This was a good thing to be able to do if you lived in the late Cretaceous period and there were hungry tyrannosaurs wandering around."

Unraveling the mysteries of pterosaur flight isn't merely an academic exercise: Texas Tech paleontologist Sankar Chatterjee has been working with aerodynamics experts to incorporate the critters' flying techniques into next-generation aircraft. So he was interested to hear about the newly published research.

"There are lots of pterosaur footprints which suggest that they walked around on four legs," Chatterjee told me. But when it came to flying, Chatterjee said giant pterosaur wings were so long that he doubted they could be unfurled fast enough after a four-footed takeoff.

"The earlier it can clear off from the ground, the better," he said. For that reason, he favors the idea that the biggest pterosaurs became airborne the way hang gliders do: by jumping off a cliff or running down an incline ... on two legs.

Habib insists that his hypothesis is more flightworthy. Detailed studies of bird takeoffs have shown that most of the power comes not from the flapping wings, but from the initial leap. "Even in the most extreme cases, they'll leap first, and then fly second," he told me.

No one can prove how pterosaurs took off, Habib said, but his findings indicate that the pterosaurs should have been able to get their wings unfolded and flapping quickly enough to keep them in the air. "I can definitively demonstrate that it's plausible that they can flap," he said.

Mark Witton, a paleontologist at the University of Portsmouth who recently identified a whole new genus of giant pterosaurs, said he sided with Habib. Here's what he told me today in an e-mail:

"The idea that pterosaurs were weather- or topography-dependent for takeoff and that they weren't strong flapping fliers - being essentially giant gliders - just doesn't make any sense. For one thing, the biggest pterosaurs, like the 500-pound critters Mike's been playing with, are often found miles and miles and miles from the nearest cliff: They occur in sediments deposited on floodplains and rivers well inland. These places also have extremely variable topography, so you can't guarantee the presence of a convenient downward slope, either.

"No, I'm in firm agreement with Mike: Being quadrupeds, pterosaurs have the capability for forelimb-assisted launch and, seeing as their most powerful muscles were associated with the forelimb, it makes sense that they would use them for takeoff if they could. In fact, Mike's research was a big relief to me: My own research was pointing to the very controversial conclusion that some of the biggest pterosaurs were massing in the 250-kilogram / 500-pound ballpark, so when he told me that he'd found a way to get such a critter into the air I was very happy.

"These animals need to be this heavy: When you're the size of a giraffe, it's just impossible that you could weigh as much as an undersize man and still function as an organism. There's simply not enough mass to allocate to the bone, muscle and other tissues. Mike's approach of thinking outside the box is absolutely correct: While it's useful to have birds and bats as modern pterosaur analogues, it's important to remember that pterosaurs are not either of these. Some aspects of their flight will be totally unique - possibly including their takeoff strategies."

You may be asking why anyone should care how pterosaurs took off. That's exactly what I asked Witton, and here's his response:

"As for its influence on the bigger pterosaur picture, this launch strategy may at least partially explain why pterosaurs managed to get so stupidly big while birds have remained comparatively small: Birds can only use their comparatively weak hindlimbs for launching, which may cap their overall size. There are other influences on the size of an animal, of course, but you can obviously only grow as large as your locomotory apparatus will allow. I guess it probably didn't have much influence on their extinction: Unique and sexy though quadrupedal launching may be, it obviously wasn't sexy enough to get pterosaurs across the Cretaceous boundary. Pterosaur extinction was therefore clearly influenced by other factors - but that's probably a whole other story (and one we don't know much about, really)."